The vessels of pressurized-water nuclear reactors have, in their upper part, a collar to which the vessel cover is fastened by means of pins engaged into holes passing through the collar of the cover and into blind internally threaded holes machined in the vessel collar. The pins are clamped by means of a set of nuts coming to bear on the upper face of the collar of the cover.
This nut/pin assembly is unscrewed and extracted from the internal threads and from the corresponding holes at certain time intervals, for the purpose of unloading and reloading the fuel and of maintaining the internal equipment.
The unloading and reloading of pressurized-water reactors normally occurs each year. This operation generally lasts for three weeks, and during this period it is expedient to protect the internal threads against introduction of foreign substances or bodies.
In most nuclear boilers, the vessel is placed inside a concrete cavity called a well. During the of the reactor, this well is emptied of water.
During the refuelling operations, the well is filled with water, in order to maintain a low radiation level in the vicinity of its upper level.
This protection, consisting of a water screen of a height of approximately eight meters, makes it possible, after removal of the cover, to extract the fuel assemblies to be replaced in the reactor core.
Likewise, this protection remains necessary for the reloading of the new assemblies in place of the spent assemblies which have been extracted. The water of the well also contains an additive in the form of boric acid, the function of which is to keep the reactor core in a subcritical state.
Before carrying out the unloading and the reloading of the fuel assemblies inside the core, it is necessary to remove the cover from the vessel. This vessel cover is usually fastened to the vessel by means of a set of pins which pass vertically through the annular collar of the cover and which engage into internally threaded blind holes located in corresponding positions on the vessel collar. There is a large number of these pins, of the order of 50 or more, depending on the type of vessel.
The vessel cover itself is removed when the well has been filled with water containing boric acid. So as to protect the internally threaded parts of the holes receiving the pins, to prevent any corrosion caused by the reactive products mixed with the water, for example boric acid, it is necessary for the bores to be protected by means of plugs.
It is known in the art to use threaded plugs which have been screwed manually into the bores. However, the fitting of these screwed plugs presents some difficulties, and the personnel responsible for these operations is therefore exposed for a considerable time to the radiation coming from the vessel.
Furthermore, the threaded plugs do not have good sealing properties, and the water containing boric acid has often been in contact with the lower part of the thread. The threaded plugs are therefore unsuitable for protecting the bore against the risks of corrosion.
To improve the sealing of the plugs introduced into the bores of the vessel collars, the sealing defects being mainly attributable to the surface irregularities of the non-threaded upper part of the bores, EP-A-0,187,707 proposed using an inflatable plug for the protection of these bores.
The plug described in this patent comprises two rigid pieces holding a U-shaped gasket. Arranged on one of the pieces is a valve making it possible to put the space defined between the two pieces and the gasket under pressure by means of compressed air. When the gasket is inflated to the desired pressure, its outer surface is in sealing contact with the non-threaded upper part of the bore to be protected. This pressure is maintained by means of a non-return valve. Leaks occur in the region of this non-return valve, and therefore the internal pressure decreases within the gasket and the requisite sealing is no longer obtained. In this case, therefore, there can also be leaks of water containing boric acid towards the inside of the bore.
Moreover, during the inflation of the plug, the air contained in the internally threaded hole is compressed, with the result that this overpressure tends to expel the plug from its receptacle.
Moreover, the upper part of the plug has an extra thickness in relation to the plane of the gasket of the vessel, thus increasing the risks that the plug will catch during the work operations. This disadvantage, combined with the overpressure generated in the bore under the plug, increases the tendency of the plug to be expelled from its receptacle.
The upper part of the plug also has a diameter larger than the diameter of the internal thread, and therefore it is not easy for the plug to be fitted remotely and there are risks that the plug will be jammed as it passes through the holes in the vessel cover.
EP-A-0029956 discloses a sealing plug for closing a threaded hole which comprises a body in two parts assembled together so as to be locked against rotation about their common axis and movable in axial translational motion relative to one another. A peripheral gasket is inserted between the two parts of the body of the plug that can be moved relatively to one another in axial translation and locked in a position in which they tighten the gasket. A tool can be connected to the plug for fitting the plug into the hole. The gasket is tightened between the two parts by a screw and nut assembly. Such a device does not allow the fitting of the plug and the tightening of the gasket to be effected correctly and remotely.